Discrete components required to realize L-C filters at low frequencies necessitate the use of heavy, bulky and expensive inductors. Recently, the development of integrated circuits has made possible the practical realization of R-C active circuits which simulate these components using generalized-immittance converters (GIC). An excellent series of articles pertaining to this subject is found in the text "Active Inductorless Filters" edited by Sanjit K. Mitra; 1971; IEEE Press, New York. In a separate article entitled: "Active Filter Design Using Generalized Impedance Converters" by Dr. L. T. Bruton and David Treleaven; Electronic Design News, Feb. 5, 1973, pp 68-75, there is described four current-conversion type GIC networks which can be used to simulate inductors or the more recently developed frequency dependent negative resistances (FDNR) which will be identified hereinafter by the symbol "D" (hence: R--FDNR = R--D). Each of these GIC networks utilizes a number of passive components and two operational amplifiers.
One of the prime considerations from both a cost and power consumption viewpoint is the total number of active elements (usually operational amplifiers) required to realize these filter elements. In an article entitled: "Economical RC Active Lossy Ladder Filters" by J. M. Rollett; Electronics Letters, Feb. 8, 1973, Vol. 9 No. 3, pp 70-72, inductive elements are simulated using only one operational amplifier. However, as the title suggests, these elements are lossy in nature and consequently cannot be used to simulate idealized lossless filters. In addition the Q of each of these circuits is related to its inductance and hence cannot be adequately controlled.
In a later article entitled: "New Active-Gyrator Circuit" by H. J. Orchard and A. N. Willson, Jun., Electronics Letters, June 27, 1974, Vol. 10 No. 13, pp 261-262; there is described an R-C active filter which uses a single operational amplifier to simulate an inductor at the grounded port when the non-grounded port is terminated by a capacitor. In a further development described in an article entitled: "Multipurpose Simulation Network With A Single Amplifier" by C. E. Schmidt and M. Lee, Electronics Letters, Vol. 11 No. 1, pp 9 and 10, there is described a simulation network which uses R-C elements in conjunction with a single operational amplifier to simulate R-Ds or L-Rs. While these circuits have several advantages over the prior circuits, they both suffer from the disadvantage that at least two impedance relationships must be met in order to simulate the various elements. Thus, both the Orchard & Willson and Schmidt & Lee networks require at least two impedance cancellations in order to realize the simulated immittances. Consequently, close matching of components is required in order to obtain good performance.